1. Field of the Invention
The present invention relates to a method for ensuring continuity of service of a personal navigation device and to an associated device hereafter also referred to as PND (“Personal Navigation Device”).
More in particular, the present invention relates to a method for ensuring continuity of service of a personal navigation device which is used in the event of insufficient reception of GNSS satellite signals and, in particular, when the device is turned on following a period of inactivity.
2. Present State of the Art
Personal navigation devices for commercial use are now widespread, which can be used while walking or driving a vehicle, and which can give the user information about the geographic position of the device itself.
Said personal navigation devices usually utilize information sent to the Earth from suitable satellites belonging to the so-called global navigation satellite systems (GNSS), which include, among others, the GPS system, the GLONASS system and the European GALILEO system, which is still under development.
Said devices suffer from the drawback that they cannot operate properly or at all if they cannot receive a sufficient number of signals from such satellites. This may occur, for example, in indoor areas and, in general, wherever the radioelectric signals emitted by the satellites cannot reach the personal navigation device because of obstacles on their path. In addition, when these devices are turned on again after having been turned off or deactivated, one must wait for the personal navigation device to execute a number of operations for acquiring satellite signals in order to calculate its own position. For a GNSS satellite system of the GPS type, such acquisition operations are called, in technical jargon, “GPS fix”.
These operations for acquiring satellite signals require a certain period of time, defined as “Time to First Fix”, which is more or less long depending on various factors, which include among others the deactivation time elapsed since the device was last turned off and the computation efficiency of the personal navigation device itself. At any rate, said period of time may vary from a few tens of seconds to a few minutes, resulting in the inconvenience that the user cannot quickly obtain from the personal navigation device any information useful for knowing his/her position and the route that must be followed to reach the desired destination.
What makes this wait even more annoying is the fact that often the user is actually perfectly aware of his/her current position, as is the case, for example, when starting for a journey. This is typically the moment when the user has just turned on the personal navigation device after a period of inactivity.
In other words, the user generally knows the starting point of the route to be planned, which may be, for example, his/her home or workplace, a certain address which was his/her previous destination, a point of interest (airport, hotel, parking lot), and the like. Also, often the starting point is located in an indoor environment (private house, public building, closed parking area, garage) or in densely populated areas (old town centres, town districts with many skyscrapers), where satellite reception is very bad or absent at all. For this reason, the user is forced to move around “blindly”, i.e. with no route indication from the personal navigation device, searching for an open place where reception of satellite localization signals is ensured, and then he/she has to wait there until satellite signals have been acquired.
Likewise, it may happen that, during a journey, the connection to the GNSS system is lost due to the presence of obstacles obscuring the satellite signals (e.g. tunnels, skyscrapers) or that the localization information becomes excessively inaccurate or unreliable because of insufficient or incomplete reception of satellite signals, so that the personal navigation device detects a current position which is wrong or anyway affected by an excessive uncertainty margin for it to be considered reliable.
In the above-mentioned cases of missed or insufficient reception of satellite signals, the personal navigation devices known in the art react in different ways.
For example, some personal navigation devices assume, when they are turned on, that the current position is the one that was detected when the device was last turned off. This assumption is wholly arbitrary and often untrue, and therefore wholly unreliable, and it is impossible for the personal navigation device to ascertain the correctness of the assumed position without first receiving a confirmation thereof through calculation of the actual position by means of a satellite fix operation.
In the event that satellite reception stops during the journey, some personal navigation devices try to deduce the current position by making some more or less plausible assumptions about the motion state of the device prior to the interruption, and cross-check such data with information obtainable from other sources, such as, for example, the available maps.
For example, when driving into a road tunnel, the personal navigation device placed on board a vehicle can no longer detect signals from satellites, and therefore assumes that the user keeps driving along the same road at the same speed as before entering the tunnel, until it receives again a valid GNSS signal which allows it to obtain the position on the basis of more reliable data.
This technique can be applied with satisfactory results only in particular cases, such as the one described, wherein the cause of the interruption of the reception of satellite signals is known a priori and the path that the personal navigation device can follow during said interruption is univocally known as well: the only unknown variable is instantaneous speed, which can hypothetically be assumed to be constant and equal to the speed detected just before entering the tunnel.
However, this technique cannot be used with acceptable results in the totality of cases. For example, when the signal is lost in a town area due to nearby buildings that limit satellite visibility, this technique cannot be applied because of the presence of many close crossings, so that the vehicle with the personal navigation device could follow various alternative routes which would be impossible to foresee a priori.
When GNSS signal reception is lost, some personal navigation devices exploit the presence of inertial systems for motion detection (gyroscopic sensors, accelerometers, and the like), called INS (“Inertial Navigation Systems”), and/or other sensors or measuring instruments (e.g. magnetometers, altimeters, odometers), which allow obtaining the instantaneous position by starting from a position known at a previous time instant and by calculating the path covered in the meantime. The set of INS systems and said other measuring instruments constitute, as a whole, a class of localization tools which are not dependent on GNSS systems; within the scope of the present description, these tools can be defined as “localization tools” in that they allow the personal navigation device to obtain its own current position, whether directly or indirectly, through suitable algorithms for correlating, processing and cross-checking the data provided by such tools.
Accelerometers belonging to the group of inertial navigation systems can also detect the spatial orientation of the apparatus or object to which they are anchored; they can thus be used for that purpose when implementing a number of functionalities, as will be described more in detail below.
Inertial navigation systems are generally based on double-integration operations carried out by starting from acceleration values measured by suitable gyroscopes; therefore, small measurement errors can accumulate over time and prejudice the accuracy of the calculated position, which rapidly falls below acceptable levels if a more reliable position datum is not available, based on which compensatory corrections can be made. This error accumulation phenomenon is commonly known as “drift error”.
It is necessary that a personal navigation device, comprising an inertial navigation system, frequently has a new absolute position obtained from the GNSS system in order to calibrate the position calculated through the inertial navigation system, otherwise the uncertainty of the position provided will quickly become intolerable as the space being travelled increases.
Therefore, also those personal navigation devices that comprise an inertial navigation system have the drawback that they can only work when an initial absolute position is known, which they cannot do without when the GNSS signal is unavailable. Because of their principle of operation, they can only calculate relative movements with respect to a known initial position. The presence of an inertial navigation system in a personal navigation device, therefore, is not at all beneficial whenever the personal navigation device does not know an initial absolute position acquired on the basis of GNSS signals, as is the case when the device is turned on after a long period of inactivity.